This application claims benefit of Japanese Patent Application Nos. 2004-130007 and 2005-056841 filed on Apr. 26, 2004 and Mar. 2, 2005, respectively, the contents of which are incorporated by the reference.
The present invention relates to liquid crystal display device and, more particularly, to an in-plane switching mode (electric field) type liquid crystal display device, comprising an array substrate, a common electrode substrate and liquid crystal sealed between the two substrates, the common electrode substrate having a plurality of post-like spacers formed on the surface opposing the array substrate and at positions dispersed in the opposing surface plane such as to keep a predetermined gap between the two substrates.
As for main terms used in this specification, their summary will be given briefly before they are described in detail with specific examples.
“Array substrate” is meant a (glass) substrate (array side substrate) formed, on the principal surface thereof, with an array of switching elements as active elements such as TFTs each disposed in the vicinity of each of the intersections between a plurality of scan lines (i.e., gate lines (or low lines) Y1, Y2, . . . , Yn), on each of which an address signal is supplied from a scan circuit, and a plurality of signal lines (i.e., data lines (column lines) X1, X2, . . . , Xm, on each of which a data signal is supplied from a hold circuit, a plurality of pixel electrodes each correspondingly connected to each of the witching elements, and common electrodes each disposed in correspondence to each of the pixel electrodes such as to generate an in-plane electric field. This substrate is also referred to as active matrix substrate or active matrix LCD glass substrate.
“Common electrode substrate” is meant a glass substrate, which is substantially parallel to, opposing and spaced apart a predetermined distance from the array substrate, has color filers (CF layers) formed on the opposing surface and seals liquid crystal between the array substrate and itself. This substrate is also referred to as opposing substrate (or opposing side substrate) or a CF substrate or a color filter substrate.
“Spacer” is meant a gap material used for providing a constant gap (i.e., LCD gap, cell gap or a gap) between the array substrate and the common electrode substrate. Sometimes, granular (or spherical) material such as “Micropearl” (Trademark) is sprinkled with a spacer sprinkler. In the technique according to the present invention, post-like spacers are provided each at a fixed position on the common electrode substrate such as to form a predetermined pattern as the distribution status.
“Black display non-uniformity irregularities” is meant a phenomenon of irregularity generation in the display due to generation of optical anisotropy in the substrate (i.e., glass) caused by deviational friction (i.e., stress having a component in the direction of deviational force exerted to the surface areas in contact with the spacers, (the deviational force being generated as a result of externally caused relative position deviation of the array substrate and the common electrode substrate from each other in the plane of the substrate). Since this phenomenon stems from residual stress in the substrate, natural recovery can not be obtained. In the in-plane electric field type liquid crystal display device, this phenomenon is particularly pronounced in the case of a display which should be a black view.
“Local gap irregularities” is meant a phenomenon that the distance (i.e., gap) between the array substrate and the common electrode substrate is not uniform in the entire substrate surface but locally varied. This phenomenon occurs in such case as when the spacers are plastically deformed by a stress exerted in excess of the limit of elasticity.
“High temperature gap irregularities (or lower part gap irregularities)” is meant a phenomenon based on the increase of the gap between the array substrate and the common electrode substrate as a result of inflation of the liquid crystal caused when the liquid crystal display device is used at a high temperature. More specifically, when the gap is increased beyond the limit of recovery from deformation of the post spacers (i.e., when the liquid crystal accommodation part is inflated), the liquid crystal is by gravitational force to gather in the neighborhood of inflated lower part of the liquid crystal display device, thus generating gap irregularities.
Liquid crystal display device find extensive applications as thin, light-weight and low power consumption flat panel displays. Among these display device is an in-lane switching mode type liquid crystal display. In this display, an in-plane electric field is generated between pixel electrodes formed on an active matrix substrate (or array side electrode) and opposing electrodes, whereby liquid crystal sealed between the active matrix substrate and the opposing substrate is caused to undergo rotation substantially horizontally in the substrate plane so as to provide the display. From this operation mode, the liquid crystal display can provide a broad sight field angle, and thus its application field is rapidly expanding.
Up to date, use has been made of a gap reducing technique to meet a demand of improving the response speed more and more. In consequence, in increasing cases post-like spacers are formed on either substrate to keep the reduced gap uniformly and accurately in the display.
However, in the case of using post-like spacers in the in-plane switching mode, display irregularities are generated in the black display. Such display irregularities are recognized particularly strongly in the black display. The display irregularities mean a phenomenon which is brought about when some externally exerted force causes a deviation of the opposing substrate with respect to the active matrix substrate in a horizontal direction (i.e., the substrate plane). In the case of post-like spacers, the frictional force between the two substrates is great compared to the case of spherical spacers. In this case, therefore, the original state before the deviation occurrence is not restored, and stress remains accumulated in the glass. This phenomenon results from resultant generation of optical anisotropy in the glass. From the sole standpoint of solving the problem of non-uniform irregularities in the black display, merely the number of the post-like spacers may be reduced to thereby reduce the frictional forces between the two substrates. However, doing so poses a different problem when a pressure is externally exerted in a direction of clamping the liquid crystal panel. The reduced number of post-like spacers can not withstand such exerted pressure, and the post-like spacers undergo plastic deformation to result in the generation of gap irregularities (these irregularities being also referred to as local gap irregularities because they are generated when the external pressure is exerted locally).
As prior art techniques for solving the above problem, mainly two methods are proposed. A first method is to increase the height of some of the post-like spacers. The increased height post-like spacers always keep the gap between the two substrates, and the remaining post-like spacers come to join the gap keeping when a pressure is exerted between the two substrates. Thus, since the number of post-like spacers keeping the gap between the two spacers at all times is reduced, the problem stemming from friction does not arise. Also, when a pressure tending to squeeze the panel is exerted, the small height post-like spacers come to join the gap support, thus suppressing the generation of local gap irregularities. The second method is to dispose post-like spacers on wiring step parts of the active matrix substrate. The parts with the post-like spacers disposed thereon keep the gap between the two substrates at all times, while parts normally not in contact with the active matrix substrate come to join the gap keeping when and only when a pressure is exerted. Thus, it is possible to expect the effects like those described above (see Literature 1: Japanese Patent Laid-Open 2002-182220, for instance).
FIG. 10 is an enlarged-scale front view showing part of the prior art liquid crystal display as described above. Each part enclosed in the solid square is a pixel 600. In each pixel, three primary color filters R, G and B are formed. Post-like spacers 304 are formed in a ratio of, for instance, one per four pixels. FIG. 11 is an enlarged-scale fragmentary sectional view showing post-like spacer parts in a prior art liquid crystal display example and the neighborhood of these parts. As shown in the sectional view of FIG. 11, this example has an array side substrate 201 and an opposing substrate 202. The opposing substrate 202 has step film 306 and protective film 303 and two different kinds of post-like spacers are formed. Large height post-like spacer 304 is formed on the step film 306, and small height post-like spacer 305 is formed on part other than the step film 306. FIG. 12 is an enlarged-scale fragmentary sectional view showing post-like spacer parts in a different prior art liquid crystal display example and the neighborhood of these parts. The FIG. 12 prior art example has an array side substrate 201 with signal lines 40 formed thereon and an opposing substrate 202. The opposing substrate 202 has post-like spacers 304 formed at position corresponding to the signal lines and other post-like spacers 305 formed at positions spaced apart from the positions of the signal lines 40. The post-like spacers formed at different positions with respect to the signal lines 40 have the following effect. The post-like spacers 304 are in contact with the signal lines 40 and supports the gap between the two substrates 201 and 202, while the other post-like spacers 305 are normally spaced apart from the array side substrate and are brought into contact therewith when and only when an external force is exerted to the opposing substrate 202.
FIGS. 16(A) to 16(C) are views for describing a prior art problem that when the liquid crystal display is used under a high temperature condition, the liquid crystal gathers in the neighborhood of the inflated lower part of the liquid crystal display. Referring to FIGS. 16(A) to 16(C), liquid crystal 500 is sealed between the array side substrate (i.e., active matrix substrate) 201 and the opposing substrate 202, and a plurality of spacers 304 are formed between the array side substrate 201 and the opposing substrate 202 to keep the gap therebetween. FIG. 16(A) shows a status at a relatively low temperature T1° C. FIG. 16(B) shows a status at a higher temperature T2° C. FIG. 16(C) shows a status at a further high temperature T3° C.
Specifically, the liquid crystal 500 is inflated with rising temperature. When the liquid crystal 500 is inflated in excess of the return extent of the post-like spacers 304 (i.e., extent of squeezing a the time of the gap formation), it is now inflated in such a manner that the post-like spacers 304 can no longer be held between the array substrate 201 and the shared electrode substrate 202 (i.e., in an increased state of the gap h between the two substrates). Eventually, as shown in FIG. 16(C), the liquid crystal 500 is collected by the gravitational force in the neighborhood of the inflated lower part of the liquid crystal display device. As a result, gap irregularities are generated. In this specification, this phenomenon is referred to as lower part gap irregularities or high temperature gap irregularities.
FIGS. 13(A) and 13(B) are views for explaining a problem in the prior art example shown in FIG. 11. In the example shown in FIG. 11, a pillow of the color material of the opposing substrate 202 and a black matrix (BM) material is provided under the post-like spacer 304. In this case, as shown in FIG. 13(A), a height (or projection extent) difference of nearly 1 μm is made between the large and small height post-like spacers 304 and 305. In such a case, as shown in FIG. 13(B), when a pressure (i.e., external force) having a component tending to reduce the gap between the array side substrate 201 and the opposing substrate 202, the post-like spacer 304 is squeezed by about slightly less than 1 μm until the post-like spacer 305 is held in contact with the array side substrate 201. The material of the post-like spacers 304 and 305 usually undergo plastic deformation. Therefore, when the pressure is released, the post-like spacer 304 is not restored up to the original height. Consequently, local gap irregularities are generated. FIGS. 14(A) to 14(C) show the manner of this generation of the local gap irregularities in the mentioned order of Figures. As shown, an excessive thickness of the step film 305 poses a problem that reduction of the friction and prevention of the generation of the local gap irregularities cannot be attained at a time.
In the other example shown in FIG. 12, the place in which the post-like spacer 304 is installed on or in contact with the array side (or active matrix) substrate 201 corresponds to a signal line (or wiring end part) 40 of the array side substrate 201, while the post-like spacer 305 is brought into contact with the array side substrate 201 when and only when a pressure for squeezing the panel is applied. FIGS. 15(A) and 15(B) are views for describing the effect on the aperture ratio in the other prior art example in FIG. 12. In the case shown in FIG. 15(A), the post-like spacer 304 is in contact with the position on the scan line 11, so that the aperture radio is not affected. In the case of FIG. 15(B), the post-like spacer 305 is located at a position deviated from the scan line 11, the signal line 40 or the frame-like black matrix 301 (i.e., the post-like spacer 305 necessarily approaches the post-like spacer 305), thus giving rise to reduction of the aperture ratio. Besides, since the post-like spacer 305 is disposed in a place close to the aperture, a problem arises that it is possible that the orientation of liquid crystal layer 500 in the neighborhood of each post-like spacer 305 is adversely affected.
Furthermore, the array side substrate 201 and the opposing substrate 202 are usually deviated from each other by several μm as engagement deviation. The fluctuations of the deviation extent leads to the problem of fluctuations of the areas in which the post-like spacers 304 and 305 are in contact with the array side substrate 201. Therefore, the intended effect is fluctuated. As yet further problems, in the prior art examples shown in FIGS. 11 and 12 two different kinds of post-like spacers, i.e., the post-like spacers 304 for keeping the gap between the two substrates 201 and 202 and the post-like spacers 305 coming to join the keeping when and only when a pressure is exerted to the panel, it is difficult to preclude the aperture ratio reduction and also obtain proper film thickness range as described above.
Moreover, the prior art has a high temperature operation problem that in this case the above lower part gap irregularities (i.e., high temperature gap irregularities) may be caused.